Build the custom ops AOT libs as C++20

[shoumikhin]

Summary

A few ExecuTorch build targets compile real PyTorch headers, instead of the small copy of c10 that ships inside this repo. ATen (the PyTorch tensor library) now needs C++20, so every one of those targets has to ask for C++20 explicitly.

Most already do: util, portable_lib, _training_lib, and the LLM runner all set CXX_STANDARD 20. Two were missed:

• custom_ops_aot_lib in extension/llm/custom_ops/CMakeLists.txt. It compiles op_sdpa_aot.cpp, which includes <torch/library.h>.
• The library built by gen_custom_ops_aot_lib in tools/cmake/Codegen.cmake. It compiles the generated RegisterSchema.cpp, which includes the same header.

Both still build at C++17 against the currently pinned PyTorch, so nothing is broken today. The problem shows up when building against a newer PyTorch, where the compile fails inside c10/util/intrusive_ptr.h, which uses std::strong_ordering, a C++20 feature:

c10/util/intrusive_ptr.h:775:8: error: 'strong_ordering' is not a member of 'std'

The fix is two lines, in the same form the other targets already use:

set_target_properties(custom_ops_aot_lib PROPERTIES CXX_STANDARD 20)

Test plan

Configured on Linux x86_64 with EXECUTORCH_BUILD_KERNELS_LLM_AOT=ON and EXECUTORCH_BUILD_KERNELS_QUANTIZED=ON, then compared the compile flags in compile_commands.json before and after the change:

target                     before     after
custom_ops                 gnu++17    gnu++17
custom_ops_aot_lib         gnu++17    gnu++20   <== changed
quantized_ops_aot_lib      gnu++17    gnu++20   <== changed
quantized_ops_lib          gnu++17    gnu++17
portable_lib               gnu++20    gnu++20
util                       gnu++20    gnu++20

Exactly the two intended targets change, and no other target is affected.

Also compiled every source file shared by these targets under C++20 to confirm none of them relies on C++17-only behavior (7 files, all pass).

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/21689

• 📄 Preview Python docs built from this PR

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❌ 2 New Failures, 3 Unrelated Failures

As of commit 44ac399 with merge base 48741ac ():

NEW FAILURES - The following jobs have failed:

• pull / test-lora-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 3d4b4dd691d10168d97d9bbd93eb78b9163e1dff8c9c0919cdc75de9056b7b8e /exec failed with exit code 1
• pull / test-lora-multimethod-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 42dd4c0b1d1420772723958ebee57d27c28f8b266a8449227ba93678a22f6e83 /exec failed with exit code 1

BROKEN TRUNK - The following jobs failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / unittest / linux / linux-job (gh) (trunk failure)
  extension/llm/modules/test/test_turboquant_kv_cache.py::TestCodebook::test_rotation_varies_with_seed
• pull / unittest-editable / linux / linux-job (gh) (trunk failure)
  extension/llm/modules/test/test_turboquant_kv_cache.py::TestCodebook::test_rotation_varies_with_seed
• pull / unittest-nxp-neutron / linux-job (gh) (trunk failure)
  test_concat_cluster_quantization__conv_7_sigmoid

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